The legislation mirrors the bill that the House of Representatives passed narrowly last month, with modest changes intended to win support from moderate senators. Meanwhile, Republican Senate leaders emphasized that the legislation is subject to change as they negotiate details in an effort to win 50 votes, the minimum required for the bill to pass.

Senate Republicans heard about the substance of the healthcare reform effort for the first time on Thursday morning, just days before the leadership intends to hold a vote.

Experts believe the legislation could leave millions of Americans without health insurance, and could have a stark impact on vulnerable populations such as people recovering from addiction, ageing middle-class baby boomers, and women and children.

That impact is likely to prompt questions about how senators can sell voters on a bill that left the House with a 17% public approval rating, and the expectation that 23 million Americans will lose health insurance.